As permanent magnet motors, there are surface permanent magnet (SPM) motors comprising permanent magnets on peripheral portions of rotors, and interior permanent magnet (IPM) motors comprising permanent magnets embedded in rotors, etc.
As shown in FIG. 16, the SPM motor has a structure in which permanent magnets 31 on a rotor surface are in direct contact with an air gap 34 between the rotor and a stator 33 comprising a yoke 32 and coils 37. The magnetic circuit shown in FIG. 16 is generally called a surface-magnet-type magnetic circuit. A magnetic flux A1 emanating from an N pole of a permanent magnet 31a penetrates the air gap 34, passes through portions 33a, 33b and 33c of the stator yoke 33, and penetrates the air gap 34 again, passes through a permanent magnet 31b and the rotor yoke 32, and returns to an S pole of the permanent magnet 31a, as shown by the arrow.
As shown in FIG. 17, the magnetic circuit of the IPM motor is called a magnet-embedded magnetic circuit or an interior-magnet-type magnetic circuit because permanent magnets 41 are embedded in a yoke 42. The yoke 42 is formed by a laminate of silicon steel sheets punched out to have magnet-shaped holes, and permanent magnets 41 are received in the holes of the yoke 42. A magnetic flux A4 emanating from an N pole of a permanent magnet 41 passes through the rotor yoke 42, penetrates an air gap 44 between a stator 43 and a rotor, successively passes through portions 43a, 43b, 43c of a stator yoke, penetrates the air gap 44 again, passes through the rotor yoke 42, and returns to an S pole of the permanent magnet 41, as shown by the arrow.
Both B1 and B2 in FIGS. 16 and 17 denote short-circuited magnetic fluxes. The magnetic fluxes B1, B2 do not act on the stator, resulting in no contribution to a torque. The magnetic fluxes B1, B2 are undesirable because they eat the magnetic flux contributing to the torque of a motor.
Many proposals were made to provide reluctance motors utilizing the saliency of soft magnetic portions in magnet rotors for a reluctance effect as shown by A5 in FIG. 17 (see Sakai, et. al., “Basic Characteristics of Permanent Magnet Reluctance Motor,” at the 1998 Meeting of the Japan Electricity Association, Lecture No. 1002). The reluctance motors are classified to switched reluctance motors and synchronous reluctance motors by stator surfaces. The switched reluctance motor generally comprises a stator having concentrated windings, and a gear-shaped rotor magnetically attracted to the teeth of the stator for rotation. The synchronous reluctance motor generally comprises a stator having distributed windings, and a rotor having one or more magnetic barriers therein. The magnetic barriers form a d-axis through which a magnetic flux easily flows, and a q-axis through which a magnetic flux does not easily flow, the difference in inductance between both axes generating reluctance torque.
Permanent magnets have drastically smaller specific permeabilities than those of soft magnetic materials such as silicon steel, etc. Utilizing the difference in specific permeability between permanent magnets and soft magnetic materials, motors having both characteristics of the permanent magnet motors and the reluctance motors can be achieved. As IPM motors, too, motors using permanent magnets as magnetic barriers to generate reluctance torque, thereby having both characteristics of the permanent magnet motors and the reluctance motors, can be achieved. Particularly because the magnet-embedded motors can effectively utilize magnetic fluxes generated by permanent magnets, they have improved efficiency at a low-speed rotation. They can also rotate up to a high-speed zone by utilizing a by-produced reluctance torque.
Magnet-embedded motors such as synchronous reluctance motors are called “reluctance permanent magnet (RPM) motors,” utilizing mainly a magnet torque and auxiliarily a reluctance torque. See W. L. Soong, T. J. E. Miller: “Practical Field-Weakening Performance of the Five Classes of Brushless Synchronous AC Motor Drive,” Proceedings of European Power Electronics Conference (1993), and W. L. Soong, D. A. Stanton, T. J. E. Miller: “Design of New Axially-Laminated Permanent Magnet Motor,” Proceedings of IEEE Industry Applications Society Annual Meeting (1993).
Such drastic improvement of the characteristics of permanent magnets provides motors with intermediate characteristics between the permanent magnet motors and the reluctance motors. Among them, permanent magnet-embedded motors are promising, because they have high efficiency and high-accuracy control, and because they can be provided with optimized characteristics for motor applications.
On the other hand, in motors widely used at present, thin plates such as silicon steel sheets, etc. having openings for permanent magnets are laminated, and constituent members are small. Accordingly, such motors are not suitable for high-speed rotation. In addition, because permanent magnets inserted into the above openings are adhered, clearance is needed to absorb working tolerance between the permanent magnets and the silicon steel sheets. This clearance acts as an air gap in a magnetic circuit, thereby lowering the efficiency of motors. Further, the clearance deteriorates the positional accuracy of the permanent magnets, resulting in uneven magnetic pole pitches and thus a cogging torque.
In addition, to lower a production cost, it is necessary to provide the permanent magnets and the silicon steel sheets with simple shapes, thereby simplifying their working. Accordingly, it is difficult to produce extremely thin portions of the permanent magnets and the silicon steel sheets with high accuracy. To effectively use a reluctance torque, however, there is an increasingly higher demand to provide irregularly shaped magnets. To solve such problems, JP 7-169633 A proposes a method for integrally molding permanent magnets and a soft magnetic material. However, this method is applicable only to the SPM motors, failing to solve the production problems of the magnet-embedded motors.
A magnet-embedded rotor needs bridging soft magnetic portions for avoiding impact and for reinforcement between pluralities of permanent magnets, but these portions permit the short-circuiting of a magnetic flux generated by the permanent magnets, resulting in a leaked magnetic flux. Accordingly, the magnetic generated by the permanent magnets cannot be fully used. To solve such problems, JP 8-331784 A proposes the construction of a yoke by a member having both magnetic portions and non-magnetic portions, and the formation of non-magnetic portions in the bridging portions. However, this technology fails to solve the above problems in working or production.
When magnet powder and soft magnetic powder are integrally compression-molded, a compression-molded body is subjected to cracking by springback at the time of removing it from a die. Even if no cracking occurred, a rotor assembled in a motor would likely be cracked by a centrifugal force if there were a weak press-bonding strength between the magnetic portions and the non-magnetic portions.
JP 2002-134311 A proposes a method for forming bonded magnets in a rotor without clearance by laminating thin plates such as silicon steel sheets, etc. having openings for receiving magnets, and injecting a compound for the bonded magnets into the openings. However, because the compound should contain a large amount of a resin (a small amount of magnet powder or iron powder) to have high flowability in this method, the resultant rotor suffers low magnetic characteristics. In addition, the larger the motor is, the more current flows in permanent magnets, resulting in an increased eddy current loss. To reduce the eddy current loss, Mita, “Eddy Current Analysis of Surface Magnet Motor,” '98 Motor Technology Symposium (1998) describes that a pole of each magnet should be divided, and that the flow of electric current should be cut by surface coatings or bonding layers. However, this method needs a lot of steps, resulting in a high production cost.